The present invention relates generally to the field of signal generators. More specifically, the present invention relates to signal generators having a phase locked loop (PLL) to tune the output signal of the signal generator.
The successful operation of modern wireless communication systems depends on the ability to produce high data rate analog and digital modulation at high frequencies (e.g., in the Gigahertz range). Meeting the needs of complex, wideband, high data rate systems requires signal generators with fine tuning capabilities and very low phase noise.
Typically, high frequency signal generators employ phase locked loops (PLLs) to stabilize the output signal. The output signal is down converted in the PLL using a digital divider to enable the use of lower frequency circuit components for phase locking the output signal to a tuning signal. The use of digital dividers to down convert the output signal suffers from the limitation that excess phase noise is introduced in the output signal. This phase noise is introduced by a factor of 20 log(N), where N is the division ratio of the digital divider.
One proposed solution to the challenge of reducing phase noise is the use of a harmonic or comb frequency generator and microwave mixer for frequency down conversion. A comb frequency generator produces a plurality of harmonics of a lower frequency (e.g., 100 MHz). An oscillator is mixed with one of the higher harmonics of the comb (e.g., 18 GHz) to produce a lower frequency output to be used in a PLL. However, this method suffers from numerous limitations. First, the comb frequency generator may introduce aliasing, making it difficult to extract the tuning signal. Second, it is generally necessary to offset the tuning signal from the comb frequency lines to provide finer tuning, which requires additional PLLs to tune within the comb frequencies (i.e., between 100 MHz xe2x80x9ctongsxe2x80x9d). Additional PLLs are expensive and complex, requiring additional microwave circuitry and shielding.
Thus, there is a need for an improved signal generator having low phase noise. There is further a need for an improved microwave signal generator that does not require the use of multiple PLLs. Further still, there is a need for a microwave signal generator having fine tuning capabilities and low phase noise with a minimal number of microwave circuits.
According to an exemplary embodiment, a signal generator includes an oscillator, a phase locked loop and a fractional divider. The oscillator is configured to provide an output signal. The phase locked loop is configured to receive the output signal and to provide a tuning signal to the oscillator. The phase locked loop has a phase detector configured to receive the output signal, to compare the output signal to a reference signal, and to provide the tuning signal to the oscillator based on the comparison. The fractional divider is outside of the phase locked loop and is configured to generate the reference signal.
According to an alternative embodiment, a microwave signal generator includes an oscillator configured to provide a microwave output signal. The signal generator further includes a phase locked loop configured to receive the microwave output signal and to provide a tuning signal to the oscillator. The phase locked loop includes a programmable frequency divider configured to receive the microwave output signal and a control signal, to select one of a plurality of division ratios based on the control signal, and to generate a divided output signal based on the selected division ratio. The phase locked loop further includes a phase detector configured to receive the divided output signal and a reference signal, to compare the divided output signal to the reference signal, and to provide the tuning signal to the oscillator based on the comparison.
According to yet another alternative embodiment, a signal generator includes a means for providing an output signal, a means for dividing the output signal based on a control signal, a means for generating a reference signal, a means for generating a tuning signal based on the divided output signal and the reference signal, and a means for tuning the output signal based on the tuning signal.
According to still another alternative embodiment, a method includes generating an output signal, dividing the output signal based on a control signal, generating a reference signal, generating a tuning signal based on the divided output signal and the reference signal, and tuning the output signal based on the tuning signal.